The subject matter disclosed herein relates to the fields of position determination, and, more specifically, to procedures for searching for signals useful in the position determination process, deriving measurements from these signals, and determining the position of an entity based on these measurements.
A GPS geo-location system is a system of earth orbiting satellites that enables a receiver of signals from the satellites to determine the position of the receiver. Each of the satellites transmits a signal that is frequency spread with a repeating pseudo-random noise (PN) code of 1,023 chips uniquely identifying the satellite. The 1,023 chips repeat every millisecond. The signal is also modulated with data bits that have a 20 ms duration.
FIG. 1 illustrates an application of the GPS geo-location system in which a receiver 100 in a wireless communications system receives transmissions from satellites 102a, 102b, 102c, 102d visible to the receiver 100. The receiver 100 derives time measurements from four or more of the transmissions. The receiver 100 provides the measurements to a position determination entity (PDE) 104, which determines the position of the receiver 100 from the measurements. Alternatively, the receiver 100 may determine its own position from this information.
The receiver 100 searches for a transmission from a particular satellite by correlating the PN code for the satellite with a received signal. The received signal is typically is a composite of transmissions from several satellites visible to the receiver 100 in the presence of noise. The correlation is performed over a range of possible shifts of the PN code. Each unique time shift is referred to as a time hypothesis. The complete set of hypotheses that are tested are referred to as a search window in time. The search window is also referred to as a search window in code space, since each offset refers to a different point within the code sequence that makes up the xe2x80x9ccode spacexe2x80x9d.
Each correlation is performed over an xe2x80x9cintegration timexe2x80x9d. The xe2x80x9cintegration timexe2x80x9d is the coherent integration time multiplied by the number of coherent integrations that are non-coherently combined.
For a particular PN code, the amount of correlation is referred to as the correlation value. If there is strong correlation between the code with which the received signal was frequency spread and the locally generated PN code, then the correlation value is high. The correlation values associated with each hypothesis define a correlation function. Peaks in the correlation function are located, and compared to a predetermined noise threshold. The threshold is selected so that the probability of falsely detecting a satellite transmission is below a predetermined level. A measurement of the relative time of arrival of the signals received from each satellite is determined by the location of the earliest peak that is above the selected threshold. It should be noted that peaks may have what is commonly referred to as side lobes. Side lobes are humps (or lower level peaks) on either side of a true peak. Such side lobes are ignored if detected.
There is a tradeoff between the accuracy and sensitivity of the search and the amount of time required to perform the search. This tradeoff is made by setting the coherent integration time, the number of coherent integrations, and the widow search size. The larger these values are, the higher the sensitivity of the receiver 100. Higher sensitivity means better detection of weak or delayed transmissions. This results in higher accuracy in the ensuing position estimates. On the other hand, if these values are larger, then a longer time is required to obtain the necessary time measurements. The risk that the receiver 100 will saturate also increases as the magnitude of these values increases.
When the transmissions from the satellites are expected to be strong, the search parameters should be set relatively low to minimize the search time. This reduces the risk that the receiver 100 with be saturated. The satellite transmissions are likely to be strong when, for example, the subscriber station is located outside on a clear day with no atmospheric or weather related disturbances. On the other hand, when the transmissions from the satellites are expected to be weak or delayed, the search parameters should be set relatively high to avoid missing weak or delayed signals. It should be clear that missing signals compromises the accuracy of the resulting position estimate. The satellite transmissions are likely to be weak or delayed when, for example, the subscriber station is located inside or there are atmospheric or weather related disturbances.
Since many subscriber stations are mobile, it cannot generally be known beforehand whether the transmissions will be strong or weak. Consequently, there is no way to determine how to set the search parameters in any particular circumstance. Accordingly, it would be advantageous to be able to set the search parameters in a way that ensures that weak signals will be detected, but that strong signals can be detected quickly an without saturating the receiver.
A method is described for searching for signals to be used in determining the location of a receiver. The method begins by conducting a first search. Measurements are derived from the results of this first search. Additional searching is avoided if the measurements satisfy one or more selected xe2x80x9cexitxe2x80x9d criteria. The position of the receiver is determined based on the measurements made from the first search.
A second search is conducted if the measurements do not satisfy the selected exit criteria. In one application, the position of the entity is then determined based on the measurements from the second search, or the combination of the first and second searches.
In one embodiment, the first search emphasizes speed rather than accuracy and sensitivity. The exit criteria are selected with the goal of ensuring that the measurements resulting from the first search are sufficient to determine the position of the entity at a desired level of accuracy. The second search is avoided if the measurements satisfy these criteria. If the measurements do not satisfy the selected criteria, then the second search is conducted. This second search emphasizes accuracy and sensitivity rather than speed.
In one implementation, the receiver is searching for signals transmitted by GPS satellites. In this implementation, all of the GPS satellites that the receiver is searching for define a xe2x80x9cfirst satellite setxe2x80x9d. Those satellites in the first satellite set for which the correlation value equals or exceeds a predetermined xe2x80x9cnoise thresholdxe2x80x9d define a xe2x80x9csecond satellite setxe2x80x9d. Those satellites in the second satellite set that have a correlation value equal to or greater than a xe2x80x9csecond thresholdxe2x80x9d define a xe2x80x9cthird satellite setxe2x80x9d. The second threshold is higher than the first threshold. A fourth set comprises all the satellites in the first set but excludes the satellites in the third set.
In one embodiment, a second search is avoided if all of the satellites in the first satellite set have a correlation value that equals or exceeds the second threshold. That is, the number of satellites in the third satellite set equals the number of satellites in the first satellite set.
In a second embodiment, the second search is avoided if a metric (i.e., a signal quality factor) determined from the azimuth angle of each satellite in the second set and the peak carrier signal to noise ratio of each such satellite exceeds a predetermined threshold.
In a third embodiment, the second search is avoided if the number of satellites in the second set equals or exceeds a predetermined threshold.
In a fourth embodiment, the second search is avoided if the peak carrier signal to noise ratio for each of the measurements in the second set equals or exceeds a predetermined threshold.
A fifth embodiment combines two or more of the four previously mentioned embodiments.